It is becoming increasingly apparent that the molecular machinery that is responsible for the epigenetic regulation of gene expression and maintenance of cell identity has fundamental roles in the development and function of the animal brain. Considering that the disruption of epigenetic processes such as DNA methylation and histone modifications can cause mental retardation, cognitive decline, addiction, and various psychiatric disorders, revealing the mechanistic connection between epigenetics, brain function, and behavior has far- reaching implications not only for neuroscience but also for human health. To better understand how epigenetic processes affect brain function, I will develop and exploit a new model system: the ant Harpegnathos saltator. Ant workers and queens exhibit starkly distinct behaviors without differences in their genetic composition; therefore, by definition, epigenetic mechanisms must contribute to activate and repress caste-specific behaviors in the appropriate individuals. Many of the ants' sophisticated behaviors are predictable, stereotypic within each caste, and often modulated by external cues that can be controlled experimentally, such as social context and chemosensory stimuli. Moreover, unlike Drosophila melanogaster, ants have a fully functional DNA methylation system. For these reasons, ants constitute an ideal experimental system to investigate epigenetics in the brain, especially in the context of social behavior. Among ants, Harpegnathos is particularly suited as a laboratory animal because any worker can, under the proper conditions, be converted into a functional pseudo-queen in a fascinating process that offers a natural experimental paradigm to study epigenetic plasticity in the adult brain and, at the same time, will allow us to develop genetic approaches that are unattainable in most other social insects. The proposed studies consist of a coordinated and ambitious investigation on the molecular mechanisms of caste determination, behavior, and social organization in ants, with particular attention to the regulation of gene expression by epigenetic processes. We will begin from an in-depth characterization of the changes in the brain transcriptome that accompany the behavioral switch from Harpegnathos workers to pseudo-queens. We will identify the gene networks that are responsible for the observed changes in phenotype and dissect the regulatory layer superimposed on these networks, with a focus on known and emerging epigenetic pathways, including DNA methylation, histone modifications, and regulation by noncoding RNAs. To address the functional significance of the genes and pathways identified, we will develop novel genetic tools in Harpegnathos and utilize already established experimental approaches such as RNA interference, pharmacological intervention, hormonal treatments, and manipulation of the social environment. This project will firmly establish Harpegnathos as a model social insect and will lay the foundations to understand at a molecular level the epigenetic regulation of brain function and social behavior.